Corrosion-resistant metal substrate

ABSTRACT

A corrosion resistant coating composition for a metal substrate is disclosed. The metal substrate, such as carbon steel, is coated with a first layer comprising a phosphate corrosion inhibitor, such as sodium phosphate monobasic (NaH 2 PO 4 ) and a second layer comprising nickel nanoparticles. In addition, an electrodeposition method for the production of the coating composition is disclosed that uses either pulse or direct current electrodeposition to form the coating composition of desired anticorrosive properties. In addition, a coated metal substrate and method for inhibiting corrosion of a metal substrate that apply the corrosion resistant coating composition in any of its embodiments are disclosed.

BACKGROUND OF THE INVENTION Technical Field

The present disclosure relates to a corrosion resistant coatingcomposition for a metal substrate, and an electrodeposition method forthe production thereof. More specifically, the present invention relatesto a metal substrate coated with a layer containing a phosphatecorrosion inhibitor and a layer containing nickel nanoparticles ofdesirable microstructure and a method employing a pulseelectrodeposition solution technique to form a coated metal substratehaving improved corrosion resistance.

Description of the Related Art

The “background” description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description which may nototherwise qualify as prior art at the time of filing, are neitherexpressly or impliedly admitted as prior art against the presentinvention.

Nickel (Ni) is commonly employed to enhance surface properties ofvarious materials using electroless nickel plating or electrolyticmethods [D. Pletcher, F. C. Walsh, Industrial Electrochemistry, 2ndedition, Chapman and Hall, London, 1990; and J. K. Dennis, T. E. Such,Nickel and Chromium Plating, Butterworth, London, 1986; A. Ul-Hamid, H.Dafalla, A. Quddus, H. Saricimen and L. M. Al-Hadhrami (2011),Electrochemical deposition of Ni on an Al—Cu alloy, J. of Mat. Eng. andPerf. doi: 10.1007/s11665-010-9816-9.—each incorporated herein byreference in its entirety]. Nickel is a preferred choice for coatingsdue to its strength and resistance to surface degradation as well as dueto its visual appeal. The electrolytic method of deposition is selectedwhen it is necessary to have some control over the crystallite size,surface morphology and orientation of Ni, which in turn cansignificantly affect the surface related properties of the coatings.Conventionally, Ni coatings have been prepared using direct current (dc)electrochemical methods [J. R. Tuck, A. M. Korsunsky, R. I. Davidson, S.J. Bull, D. M. Elliott, Surf. Coat. Technol. 127 (2000) 1; and K. C.Chan, W. K. Chan, N. S. Qu, J. Mater. Process. Technol. 89-90 (1999)447; and H. Zhao, L. Liu, J. Zhu, Y. Tang, W. Hu, Materials Letters 61(2007) 1605-1608; and A. Ul-Hamid, Abdul Quddus, F. K. Al-Yousef, A. I.Mohammed, H. Saricimen and L. M. Al-Hadhrami (December 2010)Microstructure and Surface Mechanical Properties of Electrodeposited NiCoating on Al 2014 alloy, Surface and Coatings Technology, Vol. 205 (7)pp. 2023-2030.—each incorporated herein by reference in its entirety].More recently, the use of pulse electrodeposition has become popularsince it results in Ni coatings with refined grain structure [A. M.El-Sherik, U. Erb, J. Mater. Sci. 30 (1995) 5743; and A. M. El-Sherik,U. Erb, J. Page, Surface and Coatings Technology 88 (1996) 70.—eachincorporated herein by reference in its entirety] with attractivecorrosion [R. Rofagha, R. Langer, A. M. El-Sherik, U. Erb, G. Palumbo,K. T. Aust, Scripta Metallurgica et Materialia, 25/12, 1991, 2867; andR. Mishra, R. Balasubramaniam, Corrosion Science 46 (2004) 3019; and C.Yang, Z. Yang, M. An, J. Zhang, Z. Tu, C. Li, Plat. Surf. Finish. 88 (5)(2001) 116.—each incorporated herein by reference in its entirety] andtribological properties [Y. Xuetao, W. Yu, S. Dongbai, Y. Hongying,Surface & Coatings Technology 202 (2008) 1895; and R. Mishra, B. Basu,R. Balasubramaniam, Materials Science and Engineering A 373 (2004) 370;and Y. F. Shen, W. Y. Xue, Y. D. Wang, Z. Y. Liu, L. Zuo, Surface &Coatings Technology 202 (2008) 5140; and D. H. Jeong, F. Gonzalez, G.Palumbo, K. T. Aust and U. Erb, Scripta mater. 44 (2001) 493; and LiChen, L. Wang, Z. Zeng, T. Xu, Surface & Coatings Technology 201 (2006)599; and A. Ul-Hamid, H. Dafalla, A. Quddus, H. Saricimen, L. M.Al-Hadhrami (June 2011) Microstructure and Surface Mechanical Propertiesof Pulse Electrodeposited Nickel, Applied Surface Science,doi:10.1016/j.apsusc.2011.04.120.—each incorporated herein by referencein its entirety].

Pulse plating is undertaken when current is applied in repetitive (i.e.pulse on-pulse off) square wave fashion rather than continuously as indc plating. During pulse electrodeposition, peak current density, pulseon-time, and pulse off-time are accurately controlled. Advantages ofpulse electrodeposition include its cost effectiveness and the highcurrent density, power, and range of pulse waveforms available forplating [R. J. C. Choo, J. M. Toguri, A. M. El-Sherik, U. Erb, J. Appl.Electrochem. 25 (1995) 384.—incorporated herein by reference in itsentirety]. Pulse electrodeposition results in fine nanostructuredcoating which show improvement in properties such as hardness, wear,abrasion, coefficients of friction, etc. compared to those produced byconventional dc plating.

Studies of the corrosion behavior of nanostructured coatings are ofconsiderable interest due to their potential use as protective coatingsin a wide range of applications. The fine grain structure ofnanocoatings results in a high volume fraction of intergranular defectsdue to the increased density of grain boundaries and triple junctions.There is a concern that this could have an adverse effect on thelocalized corrosion behavior of nanocoatings. However, previous studieshave shown that Ni base nanocoatings are resistant to corrosion. Somestudies report an improvement in corrosion resistance while others showbehavior comparable to polycrystalline Ni. It has also been reportedthat nanocrystalline Ni exhibits active-passive-transpassivepolarization characteristics similar to that of coarse grainedpolycrystalline Ni.

In view of the forgoing, one object of the present disclosure is toprovide a corrosion resistant coating composition for a metal substratewith an active inhibitor layer underneath nickel that can provide addedprotection from corrosion. A further aim of the present disclosure is toprovide a method for forming the corrosion resistant coating compositioncomprising electrodepositing Ni (using both direct current and pulseelectrodeposition techniques) to adhere a structurally appropriate Nilayer effectively to an inhibitor coated metal substrate. A further aimof the present disclosure is to provide a coated metal substrate as wellas a method for inhibiting the corrosion of a metal substrate thatincludes applying the coating composition described herein.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect, the present disclosure relates to acorrosion resistant coating composition for a metal substrate comprisingi) a first coating layer comprising a phosphate corrosion inhibitor andii) a second coating layer comprising nickel (Ni) nanoparticles, whereinthe first coating layer is disposed between the metal substrate and thesecond coating layer.

In one embodiment, the phosphate corrosion inhibitor is sodium phosphatemonobasic (NaH₂PO₄).

In one embodiment, the metal substrate comprises carbon steel.

In one embodiment, the nickel nanoparticles comprise fine equiaxedgrains existing in the form of colonies with an average grain size ofless than 100 nm.

In one embodiment, the second coating layer has an average thickness inthe range of 1-150 μm.

In one embodiment, the first coating layer covers greater than 75% ofthe surface of the metal substrate and the second coating layer coversgreater than 75% of the surface of the first coating layer.

In one embodiment, the second coating layer comprising Ni nanoparticleshas an instrumented nanohardness in the range of 2000-4000 MPa.

In one embodiment, the corrosion resistant coating composition imparts acorrosion rate in the range of 0.2-5.0 mils penetration per year (mpy)to the metal substrate.

In one embodiment, the corrosion resistant coating composition imparts azero current potential (ZCP) in the range of −250 mV to −600 mV to themetal substrate.

According to a second aspect, the present disclosure relates to a coatedmetal substrate comprising i) a metal substrate, ii) a first coatinglayer comprising a phosphate corrosion inhibitor, and iii) a secondcoating layer comprising nickel (Ni) nanoparticles, wherein the firstcoating layer is disposed between the metal substrate and the secondcoating layer.

According to a third aspect, the present disclosure relates to a methodof forming the corrosion resistant coating composition for a metalsubstrate comprising i) applying a phosphate corrosion inhibitor to ametal substrate to form an inhibitor coated metal substrate and ii)electrodepositing a layer of nickel nanoparticles onto the inhibitorcoated metal substrate in an electrolyte solution to form the corrosionresistant coating composition for a metal substrate.

In one embodiment, the phosphate corrosion inhibitor is sodium phosphatemonobasic (NaH₂PO₄) and the nickel nanoparticles comprise fine equiaxedgrains existing in the form of colonies with an average grain size ofless than 100 nm.

In one embodiment, the electrolyte solution is a Watt's bath comprisingi) an aqueous solution comprising 2-5 wt % NaCl relative to the totalweight of the aqueous solution, ii) nickel sulfate, iii) nickel chlorideand iv) boric acid (H₃BO₃).

In one embodiment, the electrolyte solution has a pH in the range of 3-5and the electrodepositing is performed with the electrolyte solutionhaving a temperature in the range of 35-65° C.

In one embodiment, the electrodepositing is performed by pulseelectrodeposition.

In one embodiment, the pulse electrodeposition is performed at a maximumcurrent amplitude of less than 10 A.

In one embodiment, the pulse electrodeposition comprises a repeatingsequence of current amplitude with a pulse on-time of 1-5 msec and apulse off-time of 5-15 msec.

In one embodiment, the duration of each pulse on-time is the same, theduration of each pulse off-time is the same, and the maximum currentamplitude during each pulse on-time is the same in the repeatingsequence.

According to a fourth aspect, the present disclosure relates to a methodfor inhibiting corrosion of a metal substrate comprising i) coating themetal substrate with a phosphate corrosion inhibitor to form aninhibitor coated metal surface an ii) electrodepositing a layer ofnickel nanoparticles over the inhibitor coated metal substrate in anelectrolyte solution to form a corrosion resistant metal substrate,wherein the method reduces the corrosion rate of the metal substrate inmils penetration per year (mpy) by 10-80% relative to the corrosion rateof a substantially similar metal substrate lacking the inhibitor, thelayer of nickel nanoparticles or both.

In one embodiment, the phosphate corrosion inhibitor is sodium phosphatemonobasic (NaH₂PO₄) and the nickel nanoparticles comprise fine equiaxedgrains existing in the form of colonies with an average grain size ofless than 100 nm.

The foregoing paragraphs have been provided by way of generalintroduction, and are not intended to limit the scope of the followingclaims. The described embodiments, together with further advantages,will be best understood by reference to the following detaileddescription taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a scanning electron microscopy (SEM) micrograph image showingthe surface morphology of nickel (Ni) produced by direct current (dc)electrodeposition.

FIG. 2 is a SEM micrograph image showing the surface morphology of Niproduced by pulse electrodeposition.

FIG. 3 is an atomic force microscopy (AFM) micrograph image showing thefine surface morphology of Ni produced by pulse electrodeposition.

FIG. 4 is a cross-sectional high angle annular dark field (HAADF) imagemapped by a scanning transmission electron microscope (STEM) showing Niproduced by pulse electrodeposition.

FIG. 5 is a SEM micrograph image of a carbon steel sample dipped inNaH₂PO₄—H₂O inhibitor.

FIG. 6 is an energy dispersive X-ray spectroscopy (EDX) spectrum andmicrochemical analysis of a carbon steel sample dipped in NaH₂PO₄.H₂Oinhibitor.

FIG. 7 is a SEM micrograph image showing the surface morphology of dcplated Ni covering an inhibited carbon steel sample.

FIG. 8 is a SEM micrograph image showing the surface morphology of pulseplated Ni covering an inhibited carbon steel sample.

FIG. 9 is an X-ray diffraction (XRD) spectrum of pulse electrodepositedNi covering an inhibited carbon steel sample.

FIG. 10 is a potentiodynamic polarization plot obtained for coarse Nideposited on untreated carbon steel, fine Ni deposited on untreatedcarbon steel, coarse Ni deposited on inhibitor treated carbon steel, andfine Ni deposited on inhibitor treated carbon steel.

FIG. 11 is a SEM micrograph image showing the surface morphology ofelectrodeposited Ni after polarization for coarse Ni deposited onuntreated carbon steel.

FIG. 12 is a SEM micrograph image showing the surface morphology ofelectrodeposited Ni after polarization for fine Ni deposited onuntreated carbon steel.

FIG. 13 is a SEM micrograph image showing the surface morphology ofelectrodeposited Ni after polarization for coarse Ni deposited oninhibitor treated carbon steel.

FIG. 14 is a SEM micrograph image showing the surface morphology ofelectrodeposited Ni after polarization for fine Ni deposited oninhibitor treated carbon steel.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring now to the drawings.

According to a first aspect, the present disclosure relates to acorrosion resistant coating composition for a metal substrate,comprising i) a first coating layer comprising a phosphate corrosioninhibitor and ii) a second coating layer comprising nickel (Ni)nanoparticles, wherein the first coating layer is disposed between themetal substrate and the second coating layer.

As used herein, “corrosion” refers to the process which converts refinedmetals to their more stable oxide. It is the gradual loss of a material(usually metals) by chemical reaction with their environment. Commonly,this means electrochemical oxidation of metal in reaction with anoxidant such as oxygen. Rusting, the formation of iron oxides is awell-known example of electrochemical corrosion producing oxide(s)and/or salt(s) of the original metal. Corrosion degrades the usefulproperties of materials and structures including strength, appearanceand permeability to liquids and gases. Many structural alloys corrodemerely from exposure to moisture in air, but the process can be stronglyaffected by exposure to certain substances. Corrosion can beconcentrated locally to form a pit or crack, or it can extend across awide area more or less uniformly corroding the surface. Becausecorrosion is a diffusion-controlled process, it occurs on exposedsurfaces. Thus, methods to reduce the activity of the exposed surface,such as passivation, can increase a material's corrosion resistance.

As used herein, the term “substrate” or “metal substrate” refers to ametal surface onto which a single or a plurality of materials arecoated, disposed or electrodeposited to form a coated substrate. Thesubstrate may be a non-porous starting material that becomes coated, andthe interface between the substrate and the coating material may bepore-free. Further, the bulk of the coating composition for a metalsubstrate may be the substrate, where the disposed or deposited materialforms a thin coating layer on top of the surface of the substrate.Therefore, a metal surface that has been coated with one or more layersmay also be defined as a “substrate” onto which an additional coating isadded. The substrate may be flat with no hidden surfaces. The substratemay also have a rounded or curved shape with no hidden surfaces. Thesubstrate may also be of a complex shape and have a plurality ofprotrusions and cavities with one or more hidden surfaces. A “hiddensurface” may refer to a surface that does not have a direct line ofsight and thus cannot be coated using a direct spray, stream, etc.

As used herein, “coating”, “coating layer”, “coat”, or “coated” refersto a covering that is applied to a surface of a substrate (i.e. themetal substrate and/or the inhibitor coated metal substrate). Thecoating may “substantially cover” the surface, whereby the % surfacearea coverage of the surface being coated is at least 75%, at least 80%,at least 85%, at least 90%, at least 95%, at least 95%, at least 96%, atleast 97%, at least 98%, at least 99%. In some cases, the coating may“incompletely cover”, or only cover portions of the surface beingcoated, whereby the % surface area coverage of the surface being coatedis less than 75%, less than 65%, less than 60%, less than 55%, less than50%, less than 45%, less than 40%, less than 35%, less than 30%, lessthan 25%, less than 20%, less than 15%, less than 10%. The “coating” or“coat” may refer to one material (i.e. element, metal, non-metal,nickel, phosphate corrosion inhibitor, sodium phosphate monobasic) thatcovers a surface being coated, or alternatively, the coating may referto a plurality of materials (i.e. mixtures) that cover a surface beingcoated. The plurality of materials may be applied to a surface as amixture or sequential applications of individual materials. Withsequential applications of individual materials, it may be possible toform multiple layers that are distinct from one another. These distinctlayers may have a defined interface. The term “layer” or “layers” may beused synonymously with coating or coat. The term “exterior layer” mayrefer to the coating that covers a surface of the substrate as a whole.For instance, a substrate may be covered with two distinct layers, andboth layers are referred to herein as the “external layer” and/or the“coating composition”. In the case where two or more distinct materialsare used to coat the substrate, but distinct layers are not formed, themixture of the materials in the coating may also be referred to as theexternal layer. The term “coating” may also refer to a singleapplication of a material, or a plurality of applications of the samematerial.

In a preferred embodiment, the first coating layer comprising aphosphate corrosion inhibitor substantially covers the surface of themetal substrate, where the first coating layer covers greater than 75%,preferably greater than 85%, preferably greater than 90%, preferablygreater than 95% of the surface of the metal substrate. Alternatively,the first coating may be applied to only a portion of the surface of themetal substrate (i.e. incompletely cover), and the applied coating maystill provide corrosion resistance. In a preferred embodiment, thesecond coating layer comprising nickel nanoparticles substantiallycovers the surface of the first coating layer, where the second coatinglayer covers greater than 75%, preferably greater than 85%, preferablygreater than 90%, preferably greater than 95% of the surface of thefirst coating layer. Alternatively, the second coating may be applied toonly a portion of the surface of the first coating layer (i.e.incompletely cover), and the applied coating may still provide corrosionresistance.

The coating thickness of the present disclosure may be varied dependingon the coating materials and the process for applying the coating. Inone embodiment, the average thickness of the first coating layercomprising a phosphate corrosion inhibitor is 1-150 μm, preferably 5-125μm, preferably 20-110 μm, preferably 40-100 μm, preferably 50-90 μm,preferably 60-80 μm, preferably 65-75 μm. In a preferred embodiment, theaverage thickness of the second coating layer comprising nickelnanoparticles is 1-150 μm, preferably 5-125 μm, preferably 20-110 μm,preferably 40-100 μm, preferably 50-90 μm, preferably 60-80 μm,preferably 65-75 μm. In one embodiment, the average thickness of thefirst coating layer, the second coating layer or both is uniform.Alternatively the average thickness of the first coating layer, thesecond coating layer or both may be non-uniform. The term “uniform”refers to an average coating thickness that differs by no more than 25%,by no more than 10%, by no more than 5%, by no more than 4%, by no morethan 3%, by no more than 2%, by no more than 1% at any given location onthe surface of the coated material. The term “non-uniform” refers to anaverage coating thickness that differs by more than 25% at any givenlocation on the surface of the coated material.

In one embodiment, the corrosion resistant coating composition of thepresent disclosure in any of its embodiments may impart corrosionresistance to at least one metal substrate from the exemplary groupincluding, but not limited to, copper, copper alloys (e.g. brass orbronze), aluminum, aluminum alloys (e.g. aluminum-magnesium,nickel-aluminum, aluminum-silicon), nickel, nickel alloys (e.g.nickel-titanium or nickel-chromium), iron, iron alloys, carbon steels,alloy steels, stainless steels and tool steels, preferably one or moretypes of steel, more preferably carbon steel.

Steel is an alloy of iron and carbon that is widely used in constructionand other applications because of its high tensile strength and lowcost. Carbon, other elements, and inclusions within iron act ashardening agents that prevent the movement of dislocations thatnaturally exist in the iron atom crystal lattices. The carbon in typicalsteel alloys may contribute up to 2.1% of its weight.

Steels can be broadly categorized into four groups based on theirchemical compositions: carbon steels, alloy steels, stainless steels,and tool steels. Carbon steels contain trace amounts of alloyingelements and account for 90% of total steel production. Carbon steelscan be further categorized intro three groups depending on their carboncontent: low carbon steels/mild steels contain up to 0.3% carbon, mediumcarbon steels contain 0.3-0.6% carbon, and high carbon steels containmore than 0.6% carbon. Alloys steels contain alloying elements (e.g.manganese, silicon, nickel, titanium, copper, chromium and aluminum) invarying proportions in order to manipulate the steel's properties, suchas its hardenability, corrosion resistance, strength, formability,weldability or ductility. Stainless steels generally contain between10-20% chromium as the main alloying element and are valued for highcorrosion resistance. With over 11% chromium, steel is about 200 timesmore resistant to corrosion than mild steel. These steels can be dividedinto three groups based on their crystalline structure: austeniticsteels, ferritic steels and martensitic steels. Tool steels containtungsten, molybdenum, cobalt and vanadium in varying quantities toincrease heat resistance and durability, making them ideal for cuttingand drilling equipment.

In terms of the present disclosure, the metal substrate may be steel,carbon steel, low carbon steel, mild steel, medium carbon steel, highcarbon steel, alloy steel, stainless steel, austenitic steel, ferriticsteel, martensitic steel, tool steel or mixtures thereof, mostpreferably the metal substrate is a carbon steel with a carbon contentof 0.2-1.0%.

In the present disclosure, the corrosion resistant coating compositionfor a metal substrate comprises a first coating layer comprising aphosphate corrosion inhibitor, preferably sodium phosphate monobasic(NaH₂PO₄). The metal substrate coated with the first coating layer maybe referred to as an “inhibitor coated metal substrate”.

As used herein, a “corrosion inhibitor” refers to a chemical compound orcomposition that when added to a material, typically a metal or an alloydecreases the corrosion rates of the material. The effectiveness of acorrosion inhibitor depends on fluid composition, quantity of fluid andflow regime. A common mechanism for inhibiting corrosion involvesformation of a coating, often a passivation layer, which prevents accessof the corrosive substance to the metal. The nature of a corrosioninhibitor may depend on the material being protected (most commonlymetal objects) and the corrosive agent(s) to be neutralized. Corrosiveagents may include, but are not limited to, oxygen, hydrogen sulfide andcarbon dioxide.

Generally, the mechanism of the inhibitor includes one of the following:i) the inhibitor is chemically adsorbed (chemisorption) on the surfaceof the metal and forms a protective thin film with inhibitor effect orby combination between inhibitor ions and the metallic surface, ii) theinhibitor leads a formation of a film by oxide protection of the basemetal, and/or iii) the inhibitor reacts with a potential corrosivecomponent present in media and the product is a complex. Corrosioninhibitors can be chemicals either synthetic or natural and can beclassified by i) the chemical nature as organic or inorganic, ii) themechanism of action as anodic, cathodic or an anodic-cathodic mix and byadsorption action or iii) as oxidants or not oxidants. In general, theinorganic inhibitors have cathodic mechanisms of action or anodicmechanisms of action. The organic inhibitors have both mechanisms ofaction, cathodic and anodic and afford protection by a film adsorption.

Cathodic corrosion inhibitors prevent the occurrence of the cathodicreaction of the metal. These inhibitors have metal ions able to producea cathodic reaction due to alkalinity, thus producing insolublecompounds that may precipitate selectively on cathodic sites. This maydeposit over the metal a compact and adherent film that restricts thediffusion of reducible species in these areas. This may increase theimpedance of the surface and the diffusion restriction of the reduciblespecies, that is, the oxygen diffusion and electrons conductive in theseareas. Exemplary cathodic inorganic inhibitors include, but are notlimited to, the ions of magnesium, zinc, and nickel that may react withthe hydroxyl (OH—) of water to form insoluble hydroxides (Mg(OH)₂,Zn(OH)₂, Ni(OH)₂) which are deposited on the cathodic site of the metalsurface, protecting it, polyphosphates, phosphonates, tannins, lignins,calcium salts and mixtures thereof. Anodic inhibitors (also referred toas passivation inhibitors) act by reducing anodic reaction, blocking theanode reaction and supporting the natural reaction of passivation metalsurface, they may also act by forming a film adsorbed on the metal. Ingeneral, the inhibitors react with the corrosion product, initiallyformed, resulting in a cohesive and insoluble film on the metal surface.Exemplary anodic inorganic inhibitors include, but are not limited to,nitrates, molybdates, sodium chromates, phosphates, hydroxides,silicates and mixtures thereof.

The efficiency of an organic inhibitor depends on i) chemical structureand size of the organic molecule, ii) aromaticity and/or conjugatedbonding and carbon chain length, iii) types and number of bonding atomsor groups in the molecule (either π or σ), iv) nature and the charges ofthe metal surface of adsorption mode like bonding strength to metalsubstrate, v) ability for a layer to become compact or cross-linked, vi)capability to form a complex with the atom as a solid within the metallattice and/or vii) type of the environment and adequate solubility inthe environment. Organic inhibitors, such as polymers or conductingpolymers, form an electrically insulating or chemically impermeablecoating on exposed metal surfaces that suppresses electrochemicalreactions. Inhibition of metal corrosion by organic inhibitors isinfluenced by the presence of heteroatoms (N, O and S) as the inhibitormolecules interfere with anodic or cathodic reactions occurring on themetal surfaces thus arresting or minimizing corrosion processes. Thegreater polarizability of the lone pair of electrons in the third periodelements makes them better inhibitors as a result of formation ofcoordinate-type bonds to cover and safeguard the metal surface.Corrosion inhibitors possessing N, O, or S atoms in the molecule,heterocyclic compounds and π electrons generally have hydrophilic orhydrophobic parts that are ionizable. The polar function is usuallyregarded as the reaction center for the establishment of the adsorptionprocess. The organic inhibitor that contains oxygen, nitrogen, and/orsulfur is adsorbed on the metallic surface blocking the active corrosionsites. The most effective and efficient organic inhibitors are compoundsthat have π-bonds. Exemplary organic inhibitors include, but are notlimited to, amines, urea, mercaptobenzothiazole (MBT), benzotriazole,tolyltriazole, aldehydes, heterocyclic nitrogen compounds, sulfurcontaining compounds, acetylenic compounds, ascorbic acid, succinicacid, tryptamine, caffeine, extracts of natural substances, as well asinhibitors that act in the vapor phase (i.e. volatile corrosioninhibitors) including, but not limited to, dicyclohexylammoniumbenzoate, diisopropylammonium nitrite or benzoate, ethanolamine benzoateor carbonate, the combination of urea and sodium nitrite and mixturesthereof.

In terms of the present disclosure, the first coating layer comprising acorrosion inhibitor of the corrosion resistant coating composition for ametal substrate of the present disclosure may comprise an organiccorrosion inhibitor, an inorganic corrosion inhibitor, an anodiccorrosion inhibitor, a cathodic corrosion inhibitor, a anodic-cathodicmix corrosion inhibitor and mixtures thereof, preferably an anodicinorganic corrosion inhibitor, more preferably a phosphate corrosioninhibitor.

Phosphate inhibitors can both reduce corrosion by forming a protectivelayer on the surface and can also be beneficial by inhibiting calciteprecipitation (i.e. scaling) on the surface. Sodium orthophosphate, zincorthophospnate (ZOP) and polyphosphate are commonly used corrosioninhibitors. Blends of poly/orthophosphate are also used for corrosioncontrol. Orthophosphates are generally used in the form of phosphoricacid (H₃PO₄), or neutralized orthophosphoric acid-monosodium phosphate(NaH₂PO₄), disodium phosphate (Na₂HPO₄), and trisodium phosphate(Na₃PO₄) to mitigate corrosion. Orthophosphate chemicals formpassivating films on anodic sites to suppress electrochemical reactions.In terms of the present disclosure, the phosphate corrosion inhibitor ofthe first coating layer may be orthophosphate, sodium orthophosphate,zinc orthophosphate, polyphosphate and mixtures thereof, preferablyorthophosphate or sodium orthophosphate, most preferably sodiumphosphate monobasic (NaH₂PO₄). In another embodiment, additional alkalimetals (i.e. lithium, sodium, potassium), additional alkaline earthmetals (i.e. beryllium, magnesium, calcium) and additional group 12elements (i.e. zinc, cadmium) are envisioned to serve as the counterionto the orthophosphate moiety, preferably zinc, potassium or sodium, morepreferably zinc or sodium, most preferably sodium.

In the present disclosure, the corrosion resistant coating compositionfor a metal substrate comprises a second coating layer comprising nickelnanoparticles. The metal substrate coated with the first coating layermay be referred to as an “inhibitor coated metal substrate” and theinhibitor coated metal substrate coated with the second coating layer,wherein the first coating layer is disposed between the metal substrateand the second coating layer may be referred to as the “coated metalsubstrate” or “” corrosion resistant coated metal substrate”.

Nickel (Ni, atomic number 28) is a silver/white lustrous metal with aslight golden tinge. Nickel belongs to the transition metals and is hardand ductile. Pure nickel shows a significant chemical activity that canbe observed when nickel is powdered to maximize the exposed surfacearea; larger pieces of the metal are slow to react with air at ambientconditions due to the formation of a protective oxide surface. Due tonickel's slow rate of oxidation at room temperature, it is consideredcorrosion resistant. This has led to its use in plating metals such asiron and brass, coating chemistry equipment, and manufacturing certainalloys that retain a high silvery polish. An important source of nickelis the iron ore limonite, which can contain 1-2% nickel, nickel's otherimportant ore minerals include garnierite and pentlandite. Nickel is oneof four elements that are ferromagnetic around room temperature. Themetal is chiefly valuable for the alloys it forms.

Nickel is a silver/white metal with a slight golden tinge that takes ahigh polish. The unit cell of nickel is a face-centered cube with thelattice parameter of 0.352 nm, giving an atomic radius of 0.124 nm. Thiscrystal structure is stable to pressures of at least 70 GPa. Naturallyoccurring nickel is composed of five stable isotopes; ⁵⁸Ni, ⁶⁰Ni, ⁶¹Ni,⁶²Ni and ⁶⁴Ni with ⁵⁸Ni being the most abundant (68.077% naturalabundance). Nickel occurs most often in combination with sulfur and ironin pentlandite, with sulfur in millerite, with arsenic in the mineralnickeline and with arsenic and sulfur in nickel galena. Nickel iscommonly found as the alloys kamacite and taenite. The bulk of nickelcomes from two types of ore deposits. The first are laterites, where theprincipal ore minerals are nickeliferous limonite (Fe,Ni)O(OH) andgarnierite (a hydrous nickel silicate) (Ni,Mg)₃Si₂O₅(OH)₄. The secondare magmatic sulfide deposits, where the principal ore mineral ispentlandite (Ni,Fe)₉S₈. The most common oxidation state of nickel isnickel +2, but compounds of Ni⁰, Ni⁺ and Ni³⁺ are well known, as well asexotic oxidation states Ni²⁻, Ni¹⁻, and Ni⁴⁺.

The second coating layer comprising nickel (Ni) nanoparticles refers toa nickel rich material or a deposit of substantially nickel material(i.e. greater than 60%, preferably greater than 70%, preferably greaterthan 80%, preferably greater than 90%, preferably greater than 95%). Itis further envisaged that in addition to nickel, various non-nickelmaterials (i.e. metals and non-metals) may be present in the secondcoating layer comprising nickel nanoparticles including, but not limitedto aluminum, copper, lead, iron, tin, titanium, zinc, chromium, cobalt,molybdenum, tungsten, manganese, tantalum, zirconium, niobium, rhenium,ruthenium, yttrium, vanadium, carbon, bronze, metal oxides thereof,metal sulfides thereof, calcium oxide, magnesium oxide, aluminum oxide,manganese oxide, boron, silicon, silica, sulfur, phosphorous andcombinations thereof. Additionally, nickel alloys and super alloys maybe present in the second coating layer comprising nickel nanoparticlesincluding, but not limited to, nickel-aluminum alloys, nickel-chromiumalloys, nickel-titanium alloys, Hastelloy, Inconel, Waspaloy, Renealloys, Haynes alloys, Incoloy, MP98T, TMS alloys, CMSX single crystalalloys and the like. The total weight % of these non-nickel speciesrelative to the total wt % of the second coating layer comprising nickelnanoparticles is typically no more than 40%, preferably no more than30%, preferably no more than 20%, preferably no more than 15%,preferably no more than 10%, preferably no more than 5%, preferably nomore than 4%, preferably no more than 3%, preferably no more than 2%,preferably no more than 1%.

In a preferred embodiment, the nickel nanoparticles of the presentdisclosure refers to a deposit of substantially nickel material madethrough an electroplating or electrodepositing process that coats asubstrate and/or an inhibitor coated substrate. In the presentdisclosure, particles or nanoparticles (depending on their size) thatare deposited onto the substrate may also be referred to as grains. Asused herein, grains refer to distinguishable pieces that comprise asystem, and “granular” or “granularity” describes the extent to which amaterial or system comprises distinguishable pieces or grains. It mayrefer to either the extent to which a larger entity is subdivided, orthe extent to which groups of smaller indistinguishable entities havejoined together to become larger distinguishable entities or colonies.Coarse-grained materials or systems have fewer, larger discretecomponents than fine-grained materials or systems. A coarse-graineddescription of a system regards large subcomponents while a fine-graineddescription regards smaller components of which the larger ones arecomposed. In a preferred embodiment, the nickel nanoparticles are fineequiaxed grains existing in the form of colonies that are separated bycrevices (FIG. 2). As used herein, “equiaxed” grains refer to grainsthat have axes of approximately the same length. In a preferredembodiment, the longest grain lengths vary by no more than 30%,preferably no more than 25%, preferably no more than 20%, preferably nomore than 15%, preferably no more than 10%. Ideally, the grain size ofthe nickel nanoparticles is small, especially in the initial stage ofdeposition, preferably as fine as ˜10 nm. In another embodiment, thenickel nanoparticles may be coarse compact pyramidal-shaped grains(FIG. 1) with a maximum grain size as high as 3 μm.

Nanoparticles are particles between 1 and 100 nm (10² and 10⁷ atoms) insize. A particle is defined as a small object that behaves as a wholeunit with respect to its transport and properties. The exceptionallyhigh surface area to volume ratio of nanoparticles may cause thenanoparticles to exhibit significantly different or even novelproperties from those observed in individual atoms/molecules, fineparticles and/or bulk materials. Nanoparticles can be classifiedaccording to their dimensions. Three-dimensional nanoparticles have alldimensions of less than 100 nm, and generally encompass isodimensionalnanoparticles. Examples of three dimensional nanoparticles include, butare not limited to, nanoparticles, nanospheres, nanogranules andnanobeads. Two-dimensional nanoparticles have two dimensions of lessthan 100 nm, generally including diameter. Examples of two-dimensionalnanoparticles include, but are not limited to nanotubes, nanofibers andnanowhiskers. One-dimensional nanoparticles have one dimension of lessthan 100 nm, generally thickness. Examples of one-dimensionalnanoparticles include, but are not limited to, nanosheets,nanoplatelets, nanolaminas and nanoshells. The nickel nanoparticles ofthe present disclosure are preferably three-dimensional nanoparticles,but may also be one-dimensional, two-dimensional, three-dimensional ormixtures thereof.

For substantially spherical or granular nickel nanoparticles, averageparticle size refers to the average longest linear dimension of thenickel nanoparticles and any of the length, width, height or diameter.In one embodiment, the nickel nanoparticles of the present disclosurehave an average particle size or average grain size of less than 100 nm,preferably 1-80 nm, preferably 2-70 nm, preferably 3-60 nm, preferably4-50 nm, preferably 5-40 nm, preferably 8-25 nm, preferably 10-20 nm andare considered to be “fine” grains. In another embodiment, the nickelnanoparticles and/or particles of the present disclosure may have amaximum particle size or maximum grain size of up to 5000 nm, preferablyup to 3000 nm, preferably 10-2000 nm, preferably 25-1500 nm, preferably50-1000 nm, preferably 75-500 nm, preferably 100-250 nm and areconsidered to be “coarse” grains. The size of the nickel nanoparticlesmay be a result of the technique of their formation and may impact thelevel of corrosion resistance the coating composition imparts. It isenvisaged that the size may vary from these ranges and still provideacceptable corrosion resistance.

In one embodiment, the nickel nanoparticles of the present disclosureare monodisperse, having a coefficient of variation or relative standarddeviation, expressed as a percentage and defined as the ratio of theparticle size standard deviation (a) to the particle size mean (μ)multiplied by 100 of less than 25%, preferably less than 20%, preferablyless than 15%, preferably less than 12%, preferably less than 10%,preferably less than 8%, preferably less than 6%, preferably less than5%, preferably less than 4%, preferably less than 3%, preferably lessthan 2%. In one embodiment, the nickel nanoparticles of the presentdisclosure are monodisperse having a particle size distribution rangingfrom 80% of the average particle size to 120% of the average particlesize, preferably 90-110%, preferably 95-105% of the average particlesize.

Nanoparticles are named for the real-world shapes that they appear torepresent. These morphologies sometimes arise spontaneously as an effectof the synthesis or from the innate crystallographic growth patterns ofthe materials themselves. In a preferred embodiment, the nickelnanoparticles of the present disclosure are in the form of ananoparticle or nanograins which are spherical, substantially spherical(e.g. oval, oblong, etc.) or substantially granular in shape.Alternatively, it is envisaged that the nickel nanoparticles may have amore polygonal shape and may be generally cubic or rectangular. However,the nickel nanoparticles disclosed herein may have various shapes otherthan spheres or grains and may be of any shape that provides desiredcorrosion resistant activity and/or desired properties in the resultingcorrosion resistant coating composition for a metal substrate. In apreferred embodiment, the nickel nanoparticles have a uniform granularor spherical morphology. In another embodiment, the nickel nanoparticlesmay comprise a mixture of additional morphologies (i.e. polygonal),preferably less than 10%.

In another embodiment, it is envisaged that the nickel nanoparticles maybe synthesized and formed into a variety of morphologies and formsincluding, but not limited to nanoparticles, nanogranules, nanograins,nanosheets, nanoplatelets, nanocrystals, nanospheres, nanorectangles,nanotriangles, nanopentagons, nanohexagons, nanoprisms, nanodisks,nanocubes, nanowires, nanofibers, nanoribbons, nanorods, nanotubes,nanocylinders, nanowhiskers, nanoflakes, nanofoils, nanopowders,nanoboxes, nanostars, tetrapods, nanobelts, nanourchins, nanoflowers,etc. an mixtures thereof.

Nanoparticle characterization is necessary to establish understandingand control of nanoparticle synthesis, assembly and application. In oneembodiment, the nanoparticles are characterized by at least onetechnique selected from the group consisting of electron microscopy(TEM, SEM, FESEM), powder X-ray diffraction (XRD), atomic forcemicroscopy (AFM), and energy dispersive X-ray spectroscopy (EDX). Inanother embodiment, it is envisaged that characterization is done usinga variety of other techniques. Exemplary techniques include, but are notlimited to, thermogravimetric analysis (TGA), Fourier transform infraredspectroscopy (FT-IR), ultraviolet-visible spectroscopy (UV-Vis), dynamiclight scattering (DLS), X-ray photoelectron spectroscopy (XPS), X-rayfluorescence (XRF), matrix-assisted laser desorption/ionizationtime-of-flight mass spectrometry (MALDI-TOF), Raman spectroscopy,Rutherford backscattering spectrometry (RBS), dual polarizationinterferometry, nuclear magnetic resonance (NMR) and/or mixturesthereof.

As used herein, the term nanoindentation or nanoindentation hardness isa variety of instrumented indentation hardness tests applied to smallvolumes. Indentation is one of the most commonly applied means oftesting the mechanical properties of materials. The nanoindentationtechnique was developed to measure the hardness of small volumes ofmaterial. In traditional indentation tests (macro or micro indentation),a hard tip whose mechanical properties are known (frequently comprisinga very hard material such as diamond) is pressed into a sample whoseproperties are unknown. The load placed on the indenter tip is increasedas the tip penetrates further into the specimen and soon reaches auser-defined value. At this time, the load may be held constant for aperiod or removed. The area of the residual indentation in the sample ismeasured and the hardness (H) is defined as the maximum load (P_(max))divided by the residual indentation area (A_(r)). In nanoindentation,max, small loads and tip sizes are used, so the indentation area mayonly be a few square micrometers or nanometers. Atomic force microscopyand/or scanning electron microscopy may be utilized to image theindentation. Alternatively, an indenter with a geometry known to highprecision (i.e. a Berkovich tip having a three-sided pyramid geometry)is employed and the area of the indent is determined using the knowngeometry of the indentation tip. In a preferred embodiment, the secondcoating layer comprising nickel (Ni) nanoparticles of the presentdisclosure in any of its embodiments has an instrumented nanohardness inthe range of 2000-4000 MPa, preferably 2500-3900 MPa, preferably2750-3800 MPa, preferably 3000-3700 MPa, preferably 3150-3600 MPa, morepreferably 3300-3500 MPa.

Corrosion rate is the speed at which metals undergo deterioration withina particular environment. The rate may depend on environmentalconditions and the condition or type of metal. Factors often used tocalculate or determine corrosion rate include, but are not limited to,weight loss (reduction in weight during reference time), area (initialsurface area), time (length of reference time) and density. Corrosionrate is typically computed using mils per year (mpy). Mils penetrationper year (mpy) is a unit of measurement equal to approximately onethousandth of an inch. It is used to gauge a coupon's corrosion rate.The unit is typically applied in industries like manufacturing andengineering to measure coating thickness or tolerance. Mils penetrationper year is commonly referred to as mil in the U.S. measurement system.In a preferred embodiment, the corrosion resistant coating compositionof the present disclosure in any of its embodiments imparts a corrosionrate in the range of 0.2-5.0 mils penetration per year (mpy) to themetal substrate, preferably 0.25-4.0 mpy, preferably 0.3-3.0 mpy,preferably 0.5-2.5 mpy, more preferably 0.5-2.0 mpy, more preferably0.6-1.5 mpy, more preferably 0.7-1.25 mils penetration per year (mpy).

Most metallic corrosion occurs via electrochemical reactions at theinterface between the metal and an electrolyte solution. Corrosionnormally occurs at a rate determined by an equilibrium between opposingelectrochemical reactions. One reaction is the anodic reaction, in whicha metal is oxidized, releasing electrons into the metal. The other isthe cathodic reaction, in which a solution species (often O₂ or H⁺) isreduced, removing electrons from the metal. When these two reactions arein equilibrium, the flow of electrons from each reaction is balanced,and no net electron flow (electrical current) occurs. The potential ofthe substrate or metal is the means by which the anodic and cathodicreactions are kept in balance. The equilibrium potential assumed by thesubstrate or metal in the absence of electrical connection or electricalcurrent is called the open-circuit potential, the corrosion potential,or the zero current potential (ZCP), as shown in FIG. 10. As usedherein, zero current potential refers to the equilibrium potentialassumed by a substrate at open circuit (zero current flow). In apreferred embodiment, the corrosion resistant coating composition of thepresent disclosure in any of its embodiments imparts a zero currentpotential (ZCP) in the range of −250 mV to −600 mV to the metalsubstrate, preferably −300 mV to −575 mV, preferably −350 mV to −550 mV,more preferably −375 mV to −500 mV, more preferably −400 mV to −475 mV,more preferably −405 mV to −470 mV.

Electrical impedance is the measure of opposition that a circuitpresents to a current when voltage is applied. Electrical impedancespectroscopy (EIS) has been applied to the study of corrosion systemsand been proven to be a powerful and accurate method for measuringcorrosion rates via the electrical properties of a medium as a functionof frequency. It is based on the interaction of an external field withthe electric dipole moment of the sample. The resistance of the coatingcomposition described herein, is a measurement of impedance by applyingOhm's law to a reduction in current of an applied voltage, detected inthe presence and absence of the coating composition described herein. Ina preferred embodiment, the coating composition of the presentdisclosure in any of its embodiments imparts an electrical impedance of0.5-100Ω to the metal substrate, preferably 0.75-90Ω, preferably1.0-85Ω, preferably 1.5-80Ω, preferably 2.0-75Ω, preferably 2.5-70Ω,preferably 5-65Ω, preferably 10-60, more preferably 20-50Ω, morepreferably 30-40Ω.

According to a second aspect, the present disclosure relates to a coatedmetal substrate comprising i) a metal substrate, ii) a first coatinglayer comprising a phosphate corrosion inhibitor, and iii) a secondcoating layer comprising nickel (Ni) nanoparticles wherein the firstcoating layer is disposed between the metal substrate and the secondcoating layer. In a preferred embodiment, the phosphate corrosioninhibitor is sodium phosphate monobasic (NaH₂PO₄) and the nickelnanoparticles comprise fine equiaxed grains existing in the form ofcolonies with an average grain size of less than 100 nm. In oneembodiment, the metal substrate comprises carbon steel.

According to a third aspect, the present disclosure relates to a methodof forming the corrosion resistant coating composition for a metalsubstrate of the present disclosure in one or more of its embodiments.The method comprises i) applying a coating of phosphate corrosioninhibitor to the metal substrate to form an inhibitor coated metalsubstrate and ii) electrodepositing a layer of nickel nanoparticles overthe inhibitor coated metal substrate in an electrolyte solution to formthe corrosion resistant coating composition for a metal substrate. In apreferred embodiment, the phosphate corrosion inhibitor is sodiumphosphate monobasic (NaH₂PO₄) and the nickel nanoparticles comprise fineequiaxed grains existing in the form of colonies with an average grainsize of less than 100 nm. In one embodiment, the metal substratecomprises carbon steel.

In one step of the method, a coating of phosphate corrosion inhibitor isapplied to the metal substrate to form an inhibitor coated metalsubstrate. The application of the coating of phosphate corrosioninhibitor may be achieved by dipping, brushing, spraying, painting orspin coating the metal substrate with the phosphate corrosion inhibitor.Dipping refers to a process in which an object or surface is immersed inthe inhibitor or a solution comprising the inhibitor to adhere theinhibitor to a surface of the metallic substrate, brushing refers to abristle application, spraying refers to the use of an air pressurizednozzle for dispensing the inhibitor, painting refers to the use ofrollers or spraying lacking air pressurization and spin coating refersto the use of centrifugal force in applying the phosphate corrosioninhibitor.

In a preferred embodiment, the method and the application of the coatingof phosphate corrosion inhibitor involves immersing at least one surfaceof the metal substrate in a solution comprising at least one dissolvedphosphate corrosion inhibitor. In a preferred embodiment theconcentration of phosphate corrosion inhibitor in the solution is in therange of 0.1-10 M, preferably 0.5-5 M, preferably 0.75-2.5 M, preferably1-2 M and the solution is an aqueous solution of the phosphate corrosioninhibitor. In a preferred embodiment, the immersion is carried out overa period of time in the range of 2-48 hr, preferably 8-36 hr, preferably12-32 hr, preferably 18-30 hr, preferably 20-28 hr, or 24 hr.

Cleanliness is often important to successful electroplating, sincemolecular layers, such as oil or surfactants, can prevent adhesion ofthe coating. In one embodiment, the method further comprises cleaningprocedures. These cleaning procedures may be performed on the metalsubstrate before applying a coating of phosphate corrosion inhibitor, onthe inhibitor coated metal substrate after applying a coating ofphosphate corrosion inhibitor and before electrodepositing a layer ofnickel nanoparticles, or both. Therefore, the corrosion resistantcoating composition may be substantially free of oils and/orsurfactants. Exemplary cleaning procedures include, but are not limitedto, solvent cleaning, hot alkaline detergent cleaning, electro-cleaning,acid treatment, ASTM B322 [incorporated herein by reference in itsentirety], and the like. In addition the present method may furthercomprise common industrial test for cleanliness such as the waterbreaktest, wherein the surface is thoroughly rinsed and held vertical,hydrophobic contaminants such as oils cause the water to bead and breakup, allowing the water to drain rapidly. Perfectly clean metal surfacesare hydrophilic and will retain an unbroken sheet of water that does notbead up or drain off. A version of this test is described in ASTM F22[incorporated herein by reference in its entirety]. The test does notdetect hydrophilic contaminants, but the electrodeposition process candisplace these easily as the electrolyte solutions are water-based. Inone embodiment, the method further comprises metallographicallygrounding and polishing the substrate, degreasing the substrate withacetone, and/or rinsing the substrate with water beforeelectrodepositing a layer of nickel nanoparticles.

In one step of the method, a layer of nickel nanoparticles iselectrodeposited over the inhibitor coated metal substrate in anelectrolyte solution to form the corrosion resistant coating compositionfor a metal substrate. As used herein, “electrodeposition” may refer toi) electroplating, a process that uses electric current to reducedissolved metal cations so that they may form a coherent metal coatingon an electrode, ii) electrophoretic deposition, a term for a broadrange of industrial processes including, but not limited to,electrocoating, e-coating, cathodic electrodeposition, anodicelectrodeposition, electrophoretic coating, and/or electrophoreticpainting, and iii) underpotential deposition, a phenomenon ofelectrodeposition of a species (typically reduction of a metal cation toa solid metal) at a potential less negative than the equilibrium(Nernst) potential for the reduction of this metal. In a preferredembodiment, the electrodeposition refers generally to electroplating.

As used herein “electroplating” refers to a process that uses electriccurrent to reduce dissolved metal cations so that they form a coherentmetal coating on an electrode. The term may also be used for electricaloxidation of anions onto a solid substrate. Electroplating is primarilyused to change the surface properties of an object (i.e. abrasion andwear resistance, corrosion protection, lubricity, aesthetic qualitiesand the like), but may also be used to build up thickness on undersizedparts or to form objects by electroforming. The process used inelectroplating is called electrodeposition. It is analogous to agalvanic cell acting in reverse. The part to be plated or deposited isthe cathode of the circuit. In one technique, the anode is made of themetal to be plated on the part. Both components (anode and cathode) areimmersed in a solution called an electrolyte solution containing one ormore dissolved metal salts as well as other ions that permit the flow ofelectricity, or other particles which may also coat the substrate. Apower supply supplies a current to the anode, oxidizing the metal atomsthat it comprises and allowing them to dissolve in the solution. At thecathode, the dissolved metal ions in the electrolyte solution arereduced at the interface between the solution and the cathode, such thatthey are deposited and/or “plate out” onto and coat the cathode. Therate at which the anode is dissolved is equal to the rate at which thecathode is plated, in relation to the current through the circuit. Inthe manner, the ions in the electrolyte solution or bath arecontinuously replenished by the bath. Other electroplating process mayuse a non-consumable anode such as lead or carbon. In these techniques,ions of the metal to be plated must be periodically replenished in thebath as they are drawn out of solution.

In the process of electroplating or electrodeposition the cationsassociate with the anions in the solution. These cations are reduced atthe cathode to deposit in the metallic, zero valence state. The resultis the effective transfer of the metal from the anode surface to a platecovering or acting as the cathode. The plating is most commonly a singlemetallic element, not an alloy; however, some alloys can beelectrodeposited (i.e. brass or solder). Many plating baths or solutionsinclude cyanides of other metals (i.e. potassium cyanide) in addition tocyanides of the metal to be deposited. These free cyanides facilitateanode corrosion, help to maintain a constant metal ion level andcontribute to conductivity. Additionally, non-metal chemicals such ascarbonates and phosphates may be added to increase conductivity. Whenplating or depositing is not desired on certain areas of the substrate,“stop-offs” are applied to prevent the bath from coming in contact withthe substrate. Typical stop-offs include tape, foil, lacquers, andwaxes.

In the direct current electrodeposition method, the power supplysupplies a continuous and ideally constant direct current to the anode.In the striking method, a special plating deposit termed a “strike” or“flash” may be used to form a very thin (˜ less than 0.1 μm thick)plating with high quality and good adherence to the substrate. This mayserve as a foundation for subsequent plating processes. A strike uses ahigh current density and a bath with a low ion concentration. Theprocess is slow, so more efficient plating process may be used once thedesired strike thickness is obtained. The striking method is also usedin combination with the plating of different metals. If it is desirableto plate one type of deposit onto a metal to improve corrosionresistance but this metal has inherently poor adhesion to the substrate,a strike can be first deposited that is compatible with both. In theelectrochemical deposition method, a technique is used for the growth ofmetals and conducting metal oxides. This is advantageous in that thethickness and morphology of the nanostructure can be preciselycontrolled by adjusting the electrochemical parameters, relativelyuniform and compact deposits can be synthesized in template-basedstructures, higher deposition rates are obtained and the equipment isinexpensive not requiring a high vacuum or high reaction temperature. Inthe pulse electroplating or pulse electrodeposition (PED) method theelectroplating process is modified simply. The process involves theswift alternating of the potential or current between two differentvalues resulting in a series of pulses ideally of equal amplitude,duration and polarity, separated by zero current. By changing the pulseamplitude and width, it is possible to change the deposited film'scomposition and thickness.

In the brush electroplating method, localized areas of entire items areplated using a brush saturated with plating solution. The brush,typically a stainless steel body wrapped with a cloth material that bothholds the plating solution and prevents direct contact with the itembeing plated, is connected to the positive side of a low voltage directcurrent power source, and the item to be plated is connected to thenegative. The operator may dip the brush in the plating solution, andthen apply it to the item, moving the brush continually to get an evendistribution of the plating material. Advantages of brush electroplatinginclude portability, ability to plate items that cannot be tank plated(i.e. due to a very large size), low or no masking requirements, andcomparatively low plating solution volume requirements. It does requiregreater operator involvement and may be limited in plate thickness. Inthe electroless deposition method, only one electrode and no externalsource of electric current is used. The solution must contain a reducingagent in the electroless process. In principle, any hydrogen-basedreducer can be used although the redox potential of the reducerhalf-cell must be high enough to overcome the energy barriers inherentin liquid chemistry. Typical reducers include, but are not limited to,hypophosphite (Ni) and low molecular weight aldehydes (Au, Ag, Cu). Themajor benefit of electroless deposition is the that the power sourcesand plating baths are not needed, reducing production costs, thetechnique can also plate diverse shapes and types of surfaces. It isgenerally slower and cannot create as thick of plates of metal. In termsof the present disclosure, the electrodeposition may be direct currentelectrodeposition, striking electrodeposition, electrochemicaldeposition, pulse electrodeposition, brush electrodeposition,electroless deposition or mixtures thereof; preferably theelectrodepositing is performed by direct current electrodeposition orpulse electrodeposition, most preferably pulse electrodeposition.

Nickel electrodeposition refers to technique of electrodepositing orelectroplating a thin layer of nickel or nickel nanoparticles onto ametal object. The nickel layer may be decorative, provide corrosionresistance, wear resistance, or used to build up worn or undersizedparts. The substrate is immersed into an electrolyte solution and isused as the cathode. The nickel anode is dissolved into the electrolytein the form of nickel ions, the ions travel through the solution anddeposit on the cathode.

In nickel electrodeposition, a Watt's bath can be used to deposit bothbright and semi-bright nickels as a thin layer onto a metal substrate.In a preferred embodiment, the electrolyte solution and/or theelectrochemical cell is a Watt's bath. In a preferred embodiment, theelectrolyte solution is a Watt's bath and comprises nickel sulfate(NiSO₄.6H₂O), nickel chloride (NiCl₂.6H₂O) and boric acid (H₃BO₃) in anaqueous solution, preferably an aqueous sodium chloride (NaCl) solution.In a preferred embodiment, nickel sulfate is present in the electrolytesolution or Watt's bath at a concentration of 150-300 g/L, preferably200-300 g/L, preferably 220-300 g/L, preferably 230-280 g/L, morepreferably 240-260 g/L, or 250 g/L. In a preferred embodiment, nickelchloride is present in the electrolyte solution or Watt's bath at aconcentration of 30-150 g/L, preferably 30-125 g/L, preferably 30-100g/L, preferably 30-75 g/L, more preferably 40-60 g/L, or 50 g/L. In apreferred embodiment, boric acid is present in the electrolyte solutionor Watt's bath at a concentration of 30-55 g/L, preferably 30-52 g/L,preferably 30-50 g/L, preferably 30-45 g/L, more preferably 30-40 g/L,more preferably 32.5-37.5 g/L, or 35 g/L. In a preferred embodiment, theaqueous solution is a NaCl solution comprising 2-5 wt % NaCl relative tothe total weight of the aqueous solution, preferably 2.5-4.5 wt %,preferably 3-4 wt %, more preferably 3.25-3.75 wt %, or 3.5 wt % NaClrelative to the total weight of the aqueous solution.

In a preferred embodiment, the electrolyte solution and/or Watt's bathhas a pH in the range of 3.0-5.0, preferably 3.0-4.5, preferably3.0-4.0, more preferably 3.3-3.9 or 3.6. In a preferred embodiment, theelectrodepositing is performed with the electrolyte solution and/orWatt's bath having a temperature in the range of 35-65° C., preferably40-65° C., preferably 40-60° C., preferably 40-55° C., preferably 40-50°C., more preferably 42.5-47.5° C., or 45° C. These Watt's bath componentconcentrations, as well as operating pH and temperature are exemplaryranges of operating conditions and it is equally envisaged that theseconcentrations and conditions may be varied depending on theelectrodeposition application and still provide acceptableelectrodeposition as well as acceptable corrosion resistant coatingcomposition.

It is equally envisaged that the electrolyte solution may comprisefurther additives such as brighteners. Exemplary brighteners include,but are not limited to, carrier brighteners containing sulfur andproviding uniform fine grain structure to the nickel plating (e.g.paratoluene sulfonamide or benzene sulfonic acid at 0.75-25 g/L),levelers or second class brighteners (e.g. allyl sulfonic acid orformaldehyde chloral hydrate at 0.0045-0.15 g/L), auxiliary brighteners(e.g. sodium allyl sulfonate or pyridinum propyl sulfonate at 0.075-4g/L) and inorganic brighteners (e.g. cobalt, zinc at 0.075-4 g/L). Thetype of added brightener and its concentration determine the depositappearance and luster including, but not limited to, brilliant, bright,semi-bright, satin and mixtures thereof.

In addition to a Watt's bath, it is equally envisaged that other typesof electrolyte solutions, electrochemical cells, or electrodepositingbaths may be used in the present method. Exemplary other types ofelectrolyte solutions include, but are not limited to, nickel sulfamatebaths (nickel sulfamate, Ni(SO₃NH₂)₂ at 300-450 g/L, nickel chlorideNiCl₂.6H₂O at 0-30 g/L, boric acid H₃BO₃ at 30-45 g/L), all-chloridebaths (nickel chloride, boric acid), sulfate-chloride baths, all-sulfatebaths (nickel sulfate, boric acid), hard nickel baths (nickel sulfate at160-200 g/L, ammonium chloride at 20-30 g/L, boric acid at 20-40 g/L),black nickel baths (nickel ammonium sulfate NiSO₄.(NH₄)₂SO₄.6H₂O, zincsulfate ZnSO₄ and sodium thiocyanate NaCNS) and the like.

In one embodiment, the method of forming the coating resistantcomposition comprises direct current electrodeposition or a continuousdirect current at a current density of 20-100 mA/cm², preferably 30-80mA/cm², more preferably 40-60 mA/cm², or about 50 mA/cm². The currentdensity used during the electrodepositing may be variable depending onthe application and may range from 0.01-300 mA/cm². For depositingrelatively smaller particles preferentially onto the substrate, acurrent density of 0.01-1.0 mA/cm², preferably 0.01-0.75 mA/cm², morepreferably 0.01-0.50 mA/cm² may be used. For depositing larger particlespreferentially onto the substrate, a current density of 1-300 mA/cm²,preferably 10-250 mA/cm², more preferably 20-200 mA/cm² may be used. Asused herein, the term “preferentially” refers to the deposition of aparticular particle size (e.g. small) more frequently or in higheramounts relative to a different sized particle (e.g. large), even thoughboth particle sizes are likely to be deposited onto a substrate to acertain extent.

In a preferred embodiment, the method of forming the coating resistantcomposition comprises pulse electrodeposition or pulsing current intothe electrochemical cell or electrolyte solution. In a preferredembodiment, the pulse electrodeposition is performed such that thepulsing current has a maximum amplitude of less than 10 A, preferablyless than 8 A, more preferably less than 6 A or 5 A. In a preferredembodiment, the pulsing comprises a repeating sequence of currentamplitude with a pulse on-time (or current on-time, T_(on)) of 1-ms,preferably 1-4 ms, preferably 1-3 ms, more preferably 1-2.5 ms, or 2 msand a pulse off-time (or current off-time, T_(off)) of 5-15 ms,preferably 6-14 ms, preferably 8-12 ms, more preferably 8-11 ms, morepreferably 9-11 ms, or 10 ms. In a preferred embodiment, the duration ofthe pulse or current on-times is the same and the duration of the pulseor current off-times is the same during the repeating sequence. In apreferred embodiment, the maximum current amplitude during pulseon-times is the same for each pulse on-time during the repeatingsequence (e.g. a maximum amplitude of 5 A is repeated during each pulseon-time). After and during the pulsing, the method of the presentdisclosure involves electrodepositing nickel onto the substrate andforming a deposited layer of nickel nanoparticles. Thus, the pulsingelectrodeposits nickel nanoparticles onto the substrate (i.e. theinhibitor coated metal substrate) to deposit and form the second coatinglayer comprising nickel (Ni) nanoparticles of the corrosion resistantcoating composition for a metal substrate of the present disclosure.

In one embodiment, the method may further comprise adding a gas to theelectrochemical cell or electrolyte solution or Watt's bath to promotecirculation in the solution during the pulsing and/or theelectrodepositing. The gas may include, but is not limited to, oxygen,air, or an inert gas (nitrogen, argon, etc.). In another embodiment, themethod may further comprise agitating the electrolyte solution or Watt'selectrochemical cell to provide a more uniform solution and thus a moreuniform coating, wherein the coating is a substantially uniform mixtureof the various components described herein. The agitating may include,but is not limited to stirring, sonicating, shaking, swirling and thelike. In another embodiment, the method may further comprise rotatingthe substrate about a first axis during the electrodepositing. Therotating may also involve rotating the substrate around a second axisthat is non-parallel to the first during the electrodepositing. Rotatingthe substrate may provide a coating to the entire substrate. In oneembodiment the substrate is evenly coated, where the average coatingthickness differs by no more than 5%, by no more than 4%, by no morethan 3%, preferably by no more than 2%, preferably by no more than 1% atany given location on the surface of the substrate. Without rotating thesubstrate, it may be possible to coat the substrate in an even fashionor in an uneven fashion, wherein the average thickness differs by morethan 5% at any given location on the surface of the substrate. In oneembodiment, the substrate may comprise a complex shape with at least onehidden or not easily accessible surface that is coated with thedeposited layer.

In one embodiment, the method may further comprise a variety of postprocessing procedures to aid temperature protection, creep prevention,corrosion resistance, etc. including, but not limited to, aluminizing,pack cementation, gas phase coating, chemical vapor deposition (CVD),thermal spraying, physical vapor deposition and the like. Additionalexemplary post processing procedures may include, but are not limitedto, calcining, scrubbing, acid pickling, alkaline washing, heattreating, cleaning, masking, etching, blasting treatment, grinding, UVtreatment, and the like. These post processing procedures may beperformed on the inhibitor coated metal substrate after applying acoating of phosphate corrosion inhibitor and before electrodepositing alayer of nickel nanoparticles, after formation of the formation of thecorrosion resistant coating composition for a metal substrate, or both.These techniques may be used to more adequately solidify or affix thefirst coating layer, the second coating layer, or both and may alsocomprise further treatments with other anti-corrosion materials. Inaddition, techniques to test the temperature stability, creepproperties, instrumented nanohardness, or corrosion properties may beused to test the corrosion resistant composition of the presentdisclosure after formation. Such techniques are known to those ofordinary skill in the art.

According to a fourth aspect, the present disclosure relates to a methodfor inhibiting corrosion of a metal substrate comprising i) coating themetal substrate with a phosphate corrosion inhibitor to form aninhibitor coated metal substrate and ii) electrodepositing a layer ofnickel nanoparticles over the inhibitor coated metal substrate in anelectrolyte solution to form a corrosion resistant metal substrate. In apreferred embodiment, the phosphate corrosion inhibitor is sodiumphosphate monobasic (NaH₂PO₄) and the nickel nanoparticles comprise fineequiaxed grains existing in the form of colonies with an average grainsize of less than 100 nm. In a preferred embodiment, the metal substratecomprises carbon steel. In a preferred embodiment, the method reducesthe corrosion rate of the metal substrate in mils penetration per year(mpy) by 10-80% relative to the corrosion rate of a substantiallysimilar metal substrate lacking the inhibitor, the layer of nickelnanoparticles or both, preferably 15-75%, preferably 20-70%, preferably30-65%, preferably 40-60% relative to the corrosion rate of asubstantially similar metal substrate lacking the inhibitor, the layerof nickel nanoparticles or both.

The examples below are intended to further illustrate methods andprotocols for preparing and characterizing the corrosion resistantcoatings for metal substrates of the present disclosure. Further, theyare intended to illustrate assessing the properties of these coatingcompositions. They are not intended to limit the scope of the claims.

Example 1 Electrodeposition

Plain carbon steel discs with 16 mm diameter were metallographicallyground and polished to a 1 μm surface finish. They were degreased withacetone and rinsed with distilled water. The composition of Watt's bathused for this study was NiSO₄.6H₂O (250 g), NiCl₂.6H₂O (50 g), and H₃BO₃(35 g) per liter of distilled water. Pure Ni sheet was used as anode andcarbon steel as cathode during electrodeposition. The pH and temperatureof the electrolyte was kept at 3.6 and 45° C. respectively. A currentdensity of 50 mA/cm² was used for direct current (dc) plating. Pulseelectrodeposition was performed at different peak currents of 5 A fordurations of 20 minutes. Pulse on-time (T_(oo)) and off-time (T_(off))were set at 2 and 10 msec respectively.

In some cases, inhibitor is applied to the surface of the steelsubstrate prior to inserting it into the bath for electrodeposition. Ithas been observed that the presence of an inhibitor film at the cathodesurface does not adversely affect the deposition process. The samedeposition parameters were used for treated and untreated samples anduniform, continuous and adherent deposition was obtained in each case.The inhibitor used in this study was NaH₂PO₄.H₂O whose role becomesimportant as corrosion processes progress. Its incorporation is believedto retard anodic dissolution and enhance corrosion protection ofunderlying substrate by acting as a barrier layer between the coatingand the substrate. Carbon steel substrates were immersed in NaH₂PO₄.H₂O(sodium dihydrogen phosphate monohydrate) inhibitor for a period of 24hours prior to Ni electrodeposition.

Example 2 Materials Characterization

An atomic force microscope (AFM-contact mode) and a field emissionscanning electron microscope (FE-SEM) equipped with a scanningtransmission electron microscope (STEM) detector were used to examinethe surface morphology and microstructure of electrodeposited Ni. Theinterface between the substrate and the Ni coating was imaged afterpreparing a thin cross-sectional sample using a focused ion beam (FIB)instrument. An X-ray diffractometer equipped with a monochromator wasused to determine the phase constitution, grain size and texture of Nicoatings. The diffraction spectra were generated using CuKα radiation(λ=1.54184 A°) source operating at 40 KV and 40 mA. Phase identificationwas carried out using a Bragg-Brentano (BB) configuration with θ/2θ scanaxis.

Surface morphology of Ni obtained using dc electrodeposition revealedcoarse compact pyramidal-shaped grains as show in the SEM micrograph ofFIG. 1. The maximum grain size observed was approximately 3 μm. Pulseelectrodeposition produced fine equiaxed grains existing in the form ofcolonies that are separated by crevices as shown in FIG. 2. Fine surfacemorphology of pulse electrodeposited Ni was also revealed in the AFMimage of FIG. 3. A bright field STEM cross-sectional image of theinterface between the substrate and Ni coating for a pulseelectrodeposit sample is shown in FIG. 4. It is clear that the grainsize of Ni coating is small, especially in the initial stages of thedeposition where it can be as fine as ˜10 μm. The difference in grainmorphology obtained by dc and pulse electrodeposition was also shown bya difference in their instrumented nanoindentation hardness.

Instrumented nanohardness measurements were undertaken using a Berkovich(three-sided pyramid) diamond nanoindenter which penetrated the sampleat a load of 100 mN and load/unload speed of 200 mN/min. The indenterremained stationary for 30 seconds between each loading and unloadingcycle. The nanohardness data was generated from the normal force versuspenetration depth curves. A set of four indentations was acquired foreach test. Nanoindentation hardness (H_(s)) is measured as theresistance to permanent deformation or damage and is calculated asfollow by the equation of formula (I).

$\begin{matrix}{H_{s} = {\frac{F_{\max}}{A_{p}}\mspace{14mu} ({Pascal})}} & (I)\end{matrix}$

In this formula F_(max) is the maximum force and A_(p) is the projectedcontact area. The instrumented nanohardness of Ni coatings obtained bythe dc method was 2492 MPa (230 VHN) compared to 3384 MPa (313 VHN) forpulsed Ni coatings. Refinement in grain size is thought to beresponsible for an increase in the hardness of pulsed coatings.

Carbon steel samples were dipped in NaH₂PO₄.H₂O inhibitor for 24 hoursand the top surface was examined using SEM. Inhibitor was uniformlydistributed over the surface and exhibited a spherical morphology asshown in FIG. 5. Microchemical analysis of the top surface using SEM/EDS(FIG. 6) showed the presence of Na, P and O from the inhibitor and Fefrom the underlying carbon steel substrate. The inhibited carbon steelsubstrate surfaces were deposited with Ni using dc and pulse platingtechniques. Surface morphology of dc and pulse plated Ni coveringinhibited carbon steel samples is shown in the SEM images of FIG. 7 andFIG. 8 respectively. Application of a uniform Ni coating was possibleand the presence of inhibitor on the steel surface did not seem toadversely affect the electrodeposition process.

The X-ray diffraction spectrum obtained from the top surface of thepulse Ni electrodeposited samples is shown in FIG. 9. The peak with thehighest intensity corresponds to Ni (220) indicating preferredorientation. Preferred growth orientation of planes has been reported inthe literature for pulse deposited Ni.

Example 3 Corrosion Measurements

Corrosion tests were carried out in neutral 3.5% NaCl solution at 23° C.Potentiodynamic polarization curves were acquired for a ±250 mVpotential range and at a slow scan rate of 0.166 mV s⁻¹. Athree-electrode cell was used for corrosion measurements. Saturatedcalomel electrode was used as reference and the carbon steel sampleswere employed as working electrodes. The tests were controlled through astandard potentiostat connected to a computer.

Typical potentiodynamic polarization plots obtained for dc and pulseelectrodeposited Ni with and without inhibitor treatment are shown inFIG. 10 and the various electrochemical measurements are listed inTable 1. Zero current potential (ZCP) for untreated fine grained Ni wasnoble compared to that of untreated coarse grained Ni. High grainboundary density in fine grained Ni influences the hydrogen evolutionreaction shifting the potential to more noble values. The corrosion rateof fine grained Ni was lower than the coarse grained counterpart. Thisis attributed to the more compact microstructure of the fine grained Nielectrodeposit that results in an increase in the resistance to anodicdissolution.

TABLE 1 Electrochemical measurements obtained from potentiodynamicpolarization plots for untreated and inhibitor treated Nielectrodeposits Untreated Ni (without inhibitor) Treated Ni (withinhibitor) Pulse DC Pulse DC (grain (grain size (grain (grain size size<100 nm) ~3 μm) size <100 nm) ~3 μm) ZCP −466.2 −523.5 −406.5 −569.1(mV) Corrosion rate 1.114 4.301 0.7026 4.554 (mpy)

The electrochemical behavior of inhibitor treated electrodepositsfollowed a pattern similar to that of untreated electrodeposits. Finegrained Ni exhibited a nobler ZCP and a lower corrosion rate compared tocoarse grained Ni. A comparison between the plosts reveals thatelectrodeposited Ni treated with inhibitor shows better corrosionproperties than untreated Ni. Resistance to corrosion for coarse grainedtreated coatings is comparable between treated and untreated coatings.Corrosion behavior for all four types of electrodeposits can be ratedfrom top to bottom as: treated fine grained Ni, untreated fine grainedNi, untreated coarse grained Ni, and treated coarse grained Ni. Theinfluence of grain size on the corrosion behavior was prominent in thisstudy since the layer of inhibitor existed underneath the Ni coating.

Surface morphology of untreated coarse and fine Ni after polarization isshown in FIG. 11 and FIG. 12 respectively. Density and depth of pits inthe coarse grained Ni electrodeposit is higher. The pits are shallowerand lower in density in the fine grained Ni. Surface morphologies ofcoarse and fine electrodeposits treated with an inhibitor are shown inFIG. 13 and FIG. 14 respectively. The pit density is significantly lowerin fine grained Ni. Surface damage of both treated samples is lowercompared to that of untreated samples.

Thus, the foregoing discussion discloses and describes merely exemplaryembodiments of the present invention. As will be understood by thoseskilled in the art, the present invention may be embodied in otherspecific forms without departing from the spirit or essentialcharacteristics thereof. Accordingly, the disclosure of the presentinvention is intended to be illustrative, but not limiting of the scopeof the invention, as well as other claims. The disclosure, including anyreadily discernible variants of the teachings herein, defines, in part,the scope of the foregoing claim terminology such that no inventivesubject matter is dedicated to the public.

1-9. (canceled)
 10. A corrosion resistant metal substrate, comprising: ametal substrate; a first coating layer consisting of sodium phosphatemonobasic (NaH₂PO₄), wherein the first coating layer has an averagethickness of 20-150 μm; and a second coating layer comprising nickel(Ni) nanoparticles, wherein the first and second coating layers are ofuniform thickness; wherein the first coating layer is disposed betweenthe metal substrate and the second coating layer; and wherein the nickelnanoparticles are in the form of fine equiaxed grains with an averagegrain size of less than 100 nm existing in the form of colonies that areseparated by crevices. 11-20. (canceled)
 21. The corrosion resistantmetal substrate of claim 10, wherein the metal substrate comprisescarbon steel.
 22. The corrosion resistant metal substrate of claim 10,wherein the second coating layer has an average thickness in the rangeof 1-150 μm.
 23. The corrosion resistant metal substrate of claim 10,wherein the first coating layer covers greater than 75% of the surfaceof the metal substrate and the second coating layer covers greater than75% of the surface of the first coating layer.
 24. The corrosionresistant metal substrate of claim 10, wherein the second coating layercomprising Ni nanoparticles has an instrumented nanohardness in therange of 2500-3900 MPa.
 25. The corrosion resistant metal substrate ofclaim 10, which has a corrosion rate of 0.2-4.0 mils penetration peryear (mpy).
 26. The corrosion resistant metal substrate of claim 10,which has a zero current potential (ZCP) of −350 mV to −550 mV.
 27. Thecorrosion resistant metal substrate of claim 10, wherein the secondcoating layer comprising Ni nanoparticles has an instrumentednanohardness in the range of 3300-3500 MPa.
 28. The corrosion resistantmetal substrate of claim 10, wherein the second coating layer comprisesgreater than 80% by weight of the nickel (Ni) nanoparticles relative tothe total weight of the second coating layer.